The present invention relates to a method to transfer data from a digital data modulator device to a digital data demodulator device over a digital subscriber line, said method includes, in said modulator device, the steps of modulating incoming groups of data bits into data symbols according to the trellis code modulation technique and of mapping said data symbols on carriers associated to constellation points of a constellation scheme and, in said demodulator device, the steps of de-mapping received carriers into data symbols and of de-modulating said data symbols into groups of data bits according to the trellis code de-modulation technique.
Such a method to transfer digital data is generally known in the art and may for instance be derived from the xe2x80x9cDRAFT STANDARDxe2x80x9d of ANSI xe2x80x9cT1.413-1998 Issue 2xe2x80x9d entitled xe2x80x9cNetwork and Customer Installation Interfacesxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interfacexe2x80x9d of the xe2x80x9cCOMMITTEE T1xe2x80x94TELECOMMUNICATIONSxe2x80x94Working Group T1E1.4 T1E1.4/98-007R5, Plano, Tex.; Nov. 30-Dec. 7, 1998xe2x80x9d. The invention more particularly relates to such a method used in an ADSL [Asymmetric Digital Subscriber Line] modem or any higher version thereof such as a VDSL [Very High Speed Digital Subscriber Line] modem which transfers ATM [Asynchronous Transfer Mode] cells over a ADSLNDSL network segment as mentioned in the xe2x80x9cNortel standard contributionxe2x80x9d entitled xe2x80x9cOn the ATM TC for VDSLxe2x80x9d from the author L. D. Humphrey. This standard contribution with reference xe2x80x9cTD44 Antwerpxe2x80x9d was presented on the meeting of 20-24""th April 1998 in Antwerp of ETSI""s Sub-Technical Committee TM6.
In case idle or unused ATM cells are supplied to the modem, this modem replaces the idle ATM cells with null ATM cells, i.e. cells whose header and payload sections are completely filled with zero bytes, whilst these of the idle ATM cells not always are. The modem further associates with null data symbols corresponding to the null ATM cells the origin point of a constellation diagram used to modulate carrier(s). In this way, transmitted power is reduced to a minimum during transfer of idle ATM cells and FEXT [Far End CrossTalk] is significantly reduced.
If however a relatively long sequence of idle ATM cells is transferred as a sequence of origin points of the constellation diagram, detection at the demodulator becomes difficult because of the powerless property of the origin point of the constellation diagram. In the known method, eight subsequent carriers are summed to perform the detection. Moreover, training and tracking of the Frequency EQualizer [FEQ] of the demodulator is hampered if a sequence of powerless data symbols arrives there, as a consequence of which the known modulation technique is not suitable for use in modems equipped with an adaptive FEQ whose taps are adjusted on the basis of information in the received data symbols, such as a DMT [Discrete Multi Tone] modem used in an ADSL [Asymmetric Digital Subscriber Line] or in a VDSL [Very High Speed Digital Subscriber Line] environment.
Additionally, in case the trellis code modulation is used, the correction bit(s) inserted thereby leads to create data symbols that are not always null, even for null ATM cells. Carriers associated to data symbols derived from null ATM cells can however be represented by four constellation points located in the first quadrant of the constellation diagram. Consequently, the transmission power may be relatively high during the transmission of idle ATM cells on the digital subscriber or telephone line.
An object of the present invention is to provide a method of the above known type but wherein, during the transfer of idle data over the digital subscriber line, the power consumption is minimized.
According to the invention, this object is achieved due to the fact that said method further includes, in said modulator device, the step of detecting incoming idle data, and in case idle data are detected, the step of generating null data cells derived from said idle data, and, after said steps of modulating and of mapping, the step of shifting constellation points corresponding to said null data cells towards constellation points having a relatively lower power, and in said demodulator device, the step of detecting received null data, and in case null data are detected, the step of de-shifting constellation points having the relatively lowest power towards constellation points corresponding to null data cells, prior to said steps of de-mapping and of de-modulating.
The QAM [Quadrature Amplitude Modulation] constellation points having the lowest power are the four points located each in a distinct quadrant and nearest to the origin of the constellation diagram. In this way, by associating a low power constellation point with idle data, the transmitted power is minimized during idle data transmission but no zero power points are transmitted so that it is no longer necessary to sum eight successive carriers to enable detection as mentioned above. The training and tracking of the frequency equalizer in the demodulator device is thereby simplified.
Another characterizing embodiment of the present invention is that said method further includes in said modulator device the step of randomly rotating constellation points corresponding to said null data cells amongst each other, and in said demodulator device the step of randomly de-rotating constellation points to obtain constellation points corresponding to null data cells.
Without rotating the constellation points, null data will always be mapped on the same position in the constellation diagram. By randomly rotating the constellation points of lowest power around the origin of the constellation diagram, the FEQ training and tracking is improved without increasing the transmitted power. Moreover, it is avoided that all idle data cells are represented by in-phase carriers, which would otherwise result in a high crest factor in multi-carrier systems. In other words, the Peak-to-Average Ratio [PAR] value is reduced and the FEQ updating is no longer disturbed by a constant interferer.
In a preferred embodiment, the method of the present invention is further characterized in that said constellation points are randomly rotated and randomly de-rotated over an integer multiple of 90 degrees.
It is to be noted that in the method of the present invention, the constellation points may either be shifted and then rotated or first be rotated and then shifted. In the first case, the constellation points are rotated around the origin of the constellation diagram.
The present invention also relates to a digital data modulator device and to a digital data demodulator device adapted to use the above-described method.
The digital data modulator device of the present invention comprises a trellis code modulator adapted to modulate incoming groups of data bits into data symbols according to the trellis code modulation technique, and a data encoder adapted to map said data symbols on carriers associated to constellation points of a constellation scheme.
According to the invention, the modulator device further comprises detection means adapted to detect incoming idle data, null data cell generation means adapted to generate null data cells as derived from said idle data and coupled to an input of said trellis code modulator, constellation points shifting means coupled to an output of said data encoder and adapted to move constellation points corresponding to said null data cells towards constellation points having a relatively lower power, and control means adapted to control the operation of said null data cell generation means and of said shifting means upon detection of idle data by said detection means.
In a preferred embodiment of the invention, the digital data modulator device is characterized in that it further comprises constellation points rotation means adapted to rotate amongst each other constellation points corresponding to said null data cells, the operation of said rotation means being also controlled by said control means.
On the other hand, the digital data demodulator device of the present invention comprises a data decoder adapted to translate received carriers into data symbols, and a trellis code de-modulator adapted to demodulate said data symbols into groups of data bits according to the trellis code de-modulation technique.
According to the invention, the demodulator device further comprises detection means adapted to detect received null data, constellation points de-shifting means adapted to move constellation points having the relatively lowest power towards constellation points corresponding to null data cells, and control means adapted to control the operation of said shifting means upon detection of received null data by said detection means, said constellation points de-shifting means being coupled between an input of said demodulator device and an input of said data decoder.
In a preferred embodiment of the invention, the digital data demodulator device is characterized in that it further comprises constellation points de-rotation means adapted to rotate amongst each other constellation points corresponding to null data cells, the operation of said rotation means being also controlled by said control means.
Further characterizing embodiments of the present method, modulator and demodulator devices are mentioned in the appended claims.
It is to be noticed that the term xe2x80x98comprisingxe2x80x99, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x98a device comprising means A and Bxe2x80x99 should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noticed that the term xe2x80x98coupledxe2x80x99, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x98a device A coupled to a device Bxe2x80x99 should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.